[#29] metrics: Support dump descriptions

Signed-off-by: Denis Kirillov <d.kirillov@yadro.com>

metrics/desc.go

@@ -0,0 +1,136 @@
+package metrics
+
+import (
+	"encoding/json"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+var appMetricsDesc = map[string]map[string]Description{
+	poolSubsystem: {
+		overallErrorsMetric: Description{
+			Namespace: namespace,
+			Subsystem: poolSubsystem,
+			Name:      overallErrorsMetric,
+			Help:      "Total number of errors in pool",
+		},
+		overallNodeErrorsMetric: Description{
+			Namespace:      namespace,
+			Subsystem:      poolSubsystem,
+			Name:           overallNodeErrorsMetric,
+			Help:           "Total number of errors for connection in pool",
+			VariableLabels: []string{"node"},
+		},
+		overallNodeRequestsMetric: Description{
+			Namespace:      namespace,
+			Subsystem:      poolSubsystem,
+			Name:           overallNodeRequestsMetric,
+			Help:           "Total number of requests to specific node in pool",
+			VariableLabels: []string{"node"},
+		},
+		currentErrorMetric: Description{
+			Namespace:      namespace,
+			Subsystem:      poolSubsystem,
+			Name:           currentErrorMetric,
+			Help:           "Number of errors on current connections that will be reset after the threshold",
+			VariableLabels: []string{"node"},
+		},
+		avgRequestDurationMetric: Description{
+			Namespace:      namespace,
+			Subsystem:      poolSubsystem,
+			Name:           avgRequestDurationMetric,
+			Help:           "Average request duration (in milliseconds) for specific method on node in pool",
+			VariableLabels: []string{"node", "method"},
+		},
+	},
+	stateSubsystem: {
+		healthMetric: Description{
+			Namespace: namespace,
+			Subsystem: stateSubsystem,
+			Name:      healthMetric,
+			Help:      "Current HTTP gateway state",
+		},
+		versionInfoMetric: Description{
+			Namespace:      namespace,
+			Subsystem:      stateSubsystem,
+			Name:           versionInfoMetric,
+			Help:           "Version of current FrostFS HTTP Gate instance",
+			VariableLabels: []string{"version"},
+		},
+	},
+}
+
+type Description struct {
+	Namespace      string
+	Subsystem      string
+	Name           string
+	Help           string
+	ConstantLabels []KeyValue
+	VariableLabels []string
+}
+
+type KeyValue struct {
+	Key   string `json:"key"`
+	Value string `json:"value"`
+}
+
+func (d *Description) MarshalJSON() ([]byte, error) {
+	return json.Marshal(&struct {
+		FQName         string     `json:"name"`
+		Help           string     `json:"help"`
+		ConstantLabels []KeyValue `json:"constant_labels"`
+		VariableLabels []string   `json:"variable_labels"`
+	}{
+		FQName:         d.BuildFQName(),
+		Help:           d.Help,
+		ConstantLabels: d.ConstantLabels,
+		VariableLabels: d.VariableLabels,
+	})
+}
+
+func (d *Description) BuildFQName() string {
+	return prometheus.BuildFQName(d.Namespace, d.Subsystem, d.Name)
+}
+
+func (d *Description) ConstLabelsMap() map[string]string {
+	constsLabels := make(map[string]string, len(d.ConstantLabels))
+	for _, kv := range d.ConstantLabels {
+		constsLabels[kv.Key] = kv.Value
+	}
+	return constsLabels
+}
+
+// DescribeAll returns descriptions for metrics.
+func DescribeAll() []Description {
+	var list []Description
+	for _, m := range appMetricsDesc {
+		for _, description := range m {
+			list = append(list, description)
+		}
+	}
+
+	return list
+}
+
+func newOpts(description Description) prometheus.Opts {
+	return prometheus.Opts{
+		Namespace:   description.Namespace,
+		Subsystem:   description.Subsystem,
+		Name:        description.Name,
+		Help:        description.Help,
+		ConstLabels: description.ConstLabelsMap(),
+	}
+}
+
+func newGauge(description Description) prometheus.Gauge {
+	return prometheus.NewGauge(
+		prometheus.GaugeOpts(newOpts(description)),
+	)
+}
+
+func newGaugeVec(description Description) *prometheus.GaugeVec {
+	return prometheus.NewGaugeVec(
+		prometheus.GaugeOpts(newOpts(description)),
+		description.VariableLabels,
+	)
+}